Hi, shouldnt there be possible to measure some resistance between + and ground on the alternator?
I have ordered a new one, but i'm trying to find out whats wrong with the one i have or if there is something else thats wrong. The brushes are worn out so much that there isnt any pressure on them, but they still have contact and there is 4 ohm between them. But i cant measure any ohm between + and ground, there should have been something?

Pin number 20 on the ECM is the one that goes to the gernerator field, but where does the other generator field wire from the alternator go?

 

Dude, you're always over thinking everything on your Jeep. You're driving yourself insane with all of this testing and checking. Just replace the alternator and check the new one to make sure it's working, then enjoy driving your Jeep.

 

X2^^

 

Don't worry about "internal resistance" or anything like that - that's information for the guys who are figuring out what's wrong with the thing to repair the internals.

The most you need to know is how much current your alternator can put out and still be able to maintain charging voltage. I've posted how to do just that - I don't have a like, but it's a thread I've started here, and the only tools you need would be a helper and a DMM.

 

If you want to mess with it, just put a set of brushes in it and polish the slip rings. It may work, it may not. May be a lot of work for nothing.

 

What this guy said. Also Autozone will bench test an alternator for free. It won't say its output or anything, it's just a pass/fail test.

 

It's not even a proper pass/fail.

The benches they throw those things on only test output voltage - and they only do that test at 1-2A. I've got flashlights that draw more current than that!

The idea is to maximize current draw while monitoring output voltage - you should be able to turn everything on full and still have the system voltage higher than the resting battery voltage - if the system voltage is below the battery resting voltage, then you've exceeded the output capacity of the alternator.

If you want to do a good quantitative test of the alternator, you need to spin it (either by running the engine - and knowing the draw to keep the engine running, or by using an external motor) and continue to apply a known load to it so you can know how much current you're drawing. This means that you're going to need a variable resistance pile that is rated for an awfully high power (Wattage) - so it doesn't burn up when you put the full load through it. Parts houses won't have benches built this way - but a dedicated starter/alternator shop probably will.

 

Unfortunatly, 5-90 has it right. The way I test one in the car is to turn on the rear window defroster at idle and watch the voltmeter. If it stays at 13.5 it's good to go. High beam lights and high heater blower will add much more load. My 97 ZJs alternator wouldn't carry the load of the RWD and the volts would start dropping when you came to a stop. That was my first clue I needed to buy a new one.

IMO, if you plan on adding more lights or a winch, buy a bigger one (more amps).